Soil Mechanics for Unsaturated Soils phần 8 docx

382 0.2 a 13 MEASUREMENTS OF VOLUME CHANGE INDICES Slope = - Legend 8 PP3DS 0 ST3 - (u. - u,.,) =225 kPa - Matric suction, (u, - u,) (kPa) Figure 13.18 Soil-water characteristic curve obtained from a pressure plate test on a silt com- pacted dry of optimum water content. Initial conditions 0.4 - a 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 wGs (a) Till (dry of optimum) w. = 15.2%. e, = 0.642 Silt (dry of bptimbm) I 0.6 A-A-A- a.!jB - - d 0 e, = 0.606 CI e 0.4 - . . P 0.3 - de/4wGs) or (ae/a(u, - u,))/(WG)/W, - u,))l is equivalent to the ratio of volume change indices (Le., The combined plot of Figs. 13.17, 13.18, and curve 2 [i.e., constructed from Figs. 13.18 and 13.19(a)] is de- picted in Fig. 13.20, which illustrates the volume change characteristics of an unsaturated, compacted silt. The vol- ume change indices (Le., C,, Cm, D,, and 0,) can be com- puted from Fig. 13.20. Changes in void ratio and water content due to an increase in total stress or matric suction can now be predicted using the computed volume change indices. The same test procedures were applied to other com- pacted silt and the glacial till specimens. Figure 13.19(b) summarizes the results of shrinkage tests on various com- pacted specimens. Typical volume change relationships far the compacted silt and glacial till are presented in Figs. 13.21, 13.22, and 13.23. The relationships are similar to that shown in Fig. 13.20. The computed volume change indices for the compacted silt and glacial till are tabulated in Table 13.2. These indices can be converted to other vol- ume change coefficients such as “m, and m2” or “a and b,” as explained in Chapter 12. In summary, oedometer tests, pressure plate tests, and shrinkage tests are the experiments required to obtain the volume change indices corresponding to the loading of an unsaturated soil. These tests can be performed using con- ventional soil mechanics testing procedures. The test re- sults give rise to the volume change relationships for an unsaturated soil. Cm /Dm Gs )* a 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 wGs (b) Figure 13.19 Results from shrinkage tests on compacted silts and glacial tills. (a) Shrinkage test data for the compacted silt; (b) shrinkage test data for compacted silts and glacial tills. Determination of Volume Change Indices Associated with the Transition Plane The entire void ratio constitutive surface in a semi-loga- rithmic form can be approximated by three planes, as il- lustrated in Fig. 13.24 and described in Chapter 12. The volume change indices, C, and Cm, are associated with or- 13.2 TEST PROCEDURES AND EQUIPMENTS 383 1.1 1 .o 0.9- Average initial condition: G. = 2.72, e, -0.699, w, = 16.5%. w,G, = 0.420 consolidation test results Stress state variables, (a - u.) or (u. - u,) (kPa) Figure 13.20 Volume change relationships for the silt compacted dry of optimum water content. - Till (dry of optimum) Corrected Initial -I Average initial condition: G, = 2.72, e. = 0.606, w. = 19.0%, w,G, = 0.61 6 1 0' 102 10s l(r 1V 1 07 Stress state variables, (a - UJ or (u. - u,) (kPa) Figure 13.21 Volume change relationships for the silt compacted at optimum water content. e vs (u. - u,) (curve 2) from combined pressure plate and shrinkaee test I I I A - -1 0 results 1 (curve 4) from pressure plate test results 1 10' 102 103 l(r 106 1V 1 07 Stress state variables, (a - us) or (Un - u,) (kPa) Figure 13.22 Volume charge relationships for the till compacted dry of optimum water content. 384 13 MEASUREMENTS OF VOLUME CHANGE INDICES Average initial condition: G, = 2.76, e, = 0.567, w, = 18.7%. w.G, = 0.516 from combined pressure Stress state variables, (u - u,) or (u, - u,)(kPa) Figure 13.23 Volume change relationships for the till compacted at optimum water content. thogonal planes I and 111, respectively, and can be deter- mined from the test results presented in the previous sec- tions. The volume change indices, C; and Cg, are associated with transition plane 11, and can be determined graphically as outlined in this section. The procedure is applicable to stable-structured soils. Figure 13.25 illustrates the graphical determination of the volume change indices, C; and Ch, based on the “con- stant volume” oedometer test results and the measured val- ues of the C, and Cm indices. The first step is to determine the corrected swelling pressure, PJ, as detailed in the next section. Having determined the corrected swelling pres- sure, point A in Fig. 13.25 can be plotted with a coordinate equal to (log Pi) and eo where eo is the initial void ratio. A line can be drawn through point A with a slope of C, to intersect the convergence void ratio, e* (i.e., the point where the lines converge). The second step is to determine the initial matric suction, (u, - uw)& A line can be drawn through the convergence void ratio, e*, at a slope of Cm, as shown in Fig. 13.25. The line intersects the initial void ratio line (Le., eo) at the logarithm of the initial matric suction (i.e., log The magnitudes of Pi and (u, - uw)g are used to deter- mine the location of points B, and B2 along the constant void ratio plane (Fig. 13.26). The straight line of constant void ratio on the arithmetic plot [Fig. 13.26(a)] must be conveIted to a semi-logarithmic plot, as shown in Fig. (u, - uw);). Table 13.2 A Summary of the Experimentally Measured Volumetric Deformation Indices” One-Dimensional Loading One-Dimensional Unloading Soil C, or Soil Typeb WOG~ eo DtGs cm DmGs Typeb WOG~ e0 ~1: ~rs~,d cms DmsGs DS 0.420 0.699 0.196 0.030 0.124 DS 0.419 0.702 0.040 0.126 0.033 0.263 DS 0.424 0.700 DS 0.421 0.699 os 0.516 0.606 0.177 0.082 0.158 OS 0.517 0.616 0.055 0.076 0.052 0.101 os 0.511 0.609 os 0.514 0.609 DT 0.427 0.642 0.206 0.089 0.159 DT 0.436 0.693 0.066 0.084 0.056 0.122 OT 0.516 0.567 0.179 0.106 0.171 OT 0.523 0.571 0.037 0.057 0.024 0.060 Notes: “All indices have a negative sign, as described by the sign convention in Chapter 12. b“DS” stands for silt at dry of optimum initial water content. “OS” stands for silt at optimum initial water content. “DT” stands for glacial till at dry of optimum initial water content. “OT” stands for glacial till at optimum initial water content. ‘Average slope of the unloading curve. %ope of the linear portion of the unloading curve. 13.2 TEST PROCEDURES AND EQUIPMENTS 385 & Convergence void ratio, e* 1,111 =Orthogonal planes I1 =Transition plane Figure 13.24 Approximate form for the void ratio constitutive surface on a logarithmic plot. 13.26(b). Log Pj and log (u, - u,,,): are joined by line “A,” which constitutes the chord of the asymptotic curve in Fig. 13.26(b). Line “B,” tangent to the asymptotic curve, is drawn parallel to line “A.” Line “B” intersects the log (a - u,) and log(u, - u,) axes at points BI and B2, respectively. As a result, the abscissas of points BI and B2 along the initial void ratio line are known. The third step is to draw lines extending from the con- vergence void ratio, e*, to points BI and B2 on the initial void ratio [Fig. 13.251. The slopes of these lines are equal to the C; and CA indices associated with transition plane I1 (see Fig. 13.24). The above procedure is used for obtaining the volume change indices associated with transition plane I1 on the semi-logarithmic form of the void ratio constitutive sur- face. In the arithmetic form of the constitutive surface, the volume change coefficients obtained from the extreme planes (Le., (u - u,) = 0 plane and (u, - u,) = 0 plane) ,V ,-nce void ratio, e* Arrows indicate the directions of the graphical construction Log (u - u.) or LOe (u. - uw) Figure 13.25 Graphical determination of the volume change indices. 386 13 MEASUREMENTS OF VOLUME CHANGE INDICES - (D C c s Matric suction, (u. - u,) Typical Results from Pressure Phte Tests P; =corrected swelling pressure (u. - u,E = matric suction corresponding to zero net normal stress at a constant void ratio c Legend -Actual stress path Approximated s%ss path (b) Figure 13.26 Construction of lines A and B from “constant vol- ume” oedometer test results. (a) “Constant volume” stress plane on an arithmetic scale; (b) “constant volume” stress plane on a logarithmic scale. are assumed to be applicable to every state point along a constant void ratio plane or a constant water content plane. The significance of this assumption has been explained in Chapter 12. Soil-water characteristic curves obtained from pressure plate tests are an important part of the water phase consti- tutive surface for an unsaturated soil. An unsaturated soil in the field is often subjected to more significant and fre- quent changes in matric suction, than in total stress. The soil undergoes processes of drying and wetting as a result of climatic changes. On the other hand, the applied total stress on the soil is seldom altered. Therefore, it is impor- tant to know the nature of the soil-water characteristic curve of an unsaturated soil in order to predict the water content changes when the soil is subjected to drying or wetting. Croney and Coleman (1954) have summarized soil-water characteristic curves which illustrate the different behavior observed for incompressible and compressible soils. Figure 13.27 compares the soil-water characteristic curves of soft and hard chalks, which are considered relatively incom- pressible. The drying curves of these incompressible soils exhibit essentially constant water contents at low matric suctions and rapidly decreasing water contents at higher suctions. The point where the water content starts to de- crease significantly indicates the air entry value of the soil. The data show that the hard chalk has a higher air entry value than the soft chalk. The high preconsolidation pres- sure during the formation of the hard chalk bed results in a smaller average pore size than for the soft chalk. Another noticeable characteristic is that the drying curves for both hard and soft chalks become identical at high ma- tric suctions (Fig. 13.27). This indicates that at high suc- tions, both soils have similar pore size distributions. There is a marked hysteresis between the drying and wetting curves for both soils. The effect of initial water content on the drying curves lo.’ 1 00 10’ 1 02 103 104 Matric suction, (u. - u,) (kPa) Figure 13.27 Soil-water characteristic curves for soft and hard chalks with incompressible soil structures (from Croney and Coleman, 1954). 13.2 TEST PROCEDURES AND EQUIPMENTS 387 3 0 20 60 80 100 120 140 160 Metric suction, (u. - u,) (kPa) Figure 13.28 Effects of initial water content on the drying curves of incompressible mixtures (from Croney and Coleman, 1954). of incompressible mixtures is demonstrated in Fig. 13.28. An increase in the initial water content of the soil results in a decrease in the air entry value. This can be attributed to the larger pore sizes in the high initial water content mixtures. These soils drain quickly at relatively low matric suctions. As a result, the water content in the soil with the large pores is less than the water content in the soil with small pores at matric suctions beyond the air entry value. In other words, soils with a low initial water content (or small pore sizes) require a larger matric suction value in order to commence desatumtion. There is then a slower rate of water drainage from the pores. The initial dry density of incompressible soils has a sim- ilar effect on the soil-water characteristic curve, as was illustrated by the initial water contents. As the initial dry density of an incompressible soil increases, the pore sizes are small and the air entry value of the soil is higher, as illustrated in Fig. 13.29. The highdensity specimens de- saturate at a slower rate than the low-density specimens. As a result, the high-density specimens have higher water contents than the low-density specimens at matric suctions beyond their air entry values. In addition, the hysteresis associated with the high-density specimens is less than the hysteresis exhibited by the lowdensity specimens. Croney and Coleman (1954) used the soil-water char- acteristic curve for London clay (Fig. 13.30) to illustrate the behavior of a compressible soil upon wetting and drying. The gradual decrease in water content upon drying results in the air entry value of the soil being indistinct. In this case, the shrinkage curve of the soil (Fig. 13.31) must be used together with the soil-water characteristic cume in order to determine the air entry value of the soil. The shrinkage curve clearly indicates the compressible nature of the soil. The total and water volume changes caused by an increase in matric suction are essentially equal until the water content reaches 22%. As a result, the shrinkage curve above a water content of 22% is parallel and close to the saturation line, indicating essentially a saturation condi- tion. The soil starts to desaturate when the water content goes below 22%, causing the shrinkage cume to deviate from the saturation line. The void ratio of the soil reaches I I I 10’ loD 10’ 101 Metric suction. (u. - u,.,) (kPa) Figure 13.29 Effect of initial dry density on the soil-water char- acteristic curves of a compacted silty sand (from Cronev and Coleman, 1954). 388 13 MEASUREMENTS OF VOLUME CHANGE INDICES 10-1 IOD 10’ 102 103 104 106 108 Figure 13.30 Soil-water characteristic curves for London clay (from Croney and Coleman, 1954). Matric suction, (u. - u,) (kPa) a limiting value (Le., e = 0.48), corresponding to a water content of 0%. A water content of 22 % corresponds to a matric suction in the natural soil of approximately lo00 kPa, as indicated by the soil-water characteristic curve (Fig. 13.30). Some irreversible structural changes causing an irrever- sible volume change occur primarily during the first drying process, as indicated by curve A in Fig. 13.30. Subsequent wetting and drying cycles follow curves B and C (Fig. 13.30), respectively. Curves B and C have lower water contents than curve A, with the difference indicating irre- versible volume change. Curve D in Fig. 13.30 was obtained from initially slur- 0.9r1 I I I I I I i Water content, w (%I Figure 13.31 Shrinkage curve for London clay (from Croney and Coleman, 1954). ried specimens where the soil structure was partially dis- turbed. Curve A for the natural soil joined curve D at a matric suction of 6300 ma, indicating the maximum suc- tion to which the clay has been subjected during its geo- logical history. The maximum suction has a similar mean- ing to the preconsolidation pressure of a saturated soil, and this is explained in further detail in Chapter 14. The devia- tions of the natural soil curves A, B, and C from the ini- tially slurried soil curve D represent the natural state of disturbance due to past drying and wetting cycles. Another curve plotted in Fig. 13.30 is curve G that re- lates the water content to the matric suction for disturbed soil specimens. Curve G is not a soil-water characteristic curve since the points on the curve were obtained from soil specimens with different soil structures. A soil-water char- acteristic curve must be obtained from a single specimen or several specimens with “identical” initial soil struc- tures. Curve G appears to be unique for London clay, re- gardless of the matric suction of the soil. State points along the drying curve D or curve A will move to corresponding points on curve G when disturbed at a constant water con- tent. Similarly, state points along any wetting curve will move to curve G when disturbed at a constant water con- tent. Disturbance can take the form of remolding or thor- oughly mixing the specimens. Similar relationships to curve G have also been found for other soils (see Fig. 13.7). It can therefore be concluded that there is a unique relation- ship between water content and matric suction for a dis- turbed soil, regardless of its soil structure, initial matric suction, or its initial state path (Croney and Coleman, 1954). A similar relationship is commonly observed in soils compacted at various water contents and dry densities (see Chapter 4). In other words, compacted soils have a unique relationship between water content and matric suction, re- gardless of the compacted dry density of the specimens. Determination of In Situ Stress State Using Oedometer Test Results One-dimensional oedometer tests are most often used for the assessment of the in situ stress state and the swelling properties of expansive, unsaturated soils. The oedometer can only be used to perform testing in the net normal stress plane. Therefore, the assumption is made that it is possible to eliminate the matric suction from the soil and obtain the necessary soil properties and stress state values from the net normal stress plane. The “free-swell” and “constant volume” tests (Fig. 13.2) are two commonly used proce- dures which first eliminate the soil matric suction. ‘ ‘Constant Volume ’ ’ Test Let us first consider the “constant volume” oedometer test. In this procedure, the specimen is subjected to a token load and submerged in water. The release of the negative pore- water pressure to atmospheric conditions results in a ten- 13.2 TEST PROCEDURES AND EQUIPMENTS 389 dency for the specimen to swell. As the specimen tends to swell, the applied load is increased to maintain the speci- men at a constant volume. This procedure is continued un- til the specimen exhibits no further tendency to swell. The applied load at this point is refed to as the “uncorrected swelling pressure,” P, . The specimen is then further loaded and unloaded in a conventional manner. The test results are generally plotted as shown in Fig. 13.2(b). The actual stress paths followed during the test can be more fully understood by use of a three-dimensional plot, with each of the stress state variables forming an ab- scissa (Fig. 13.32). It is important to understand the stress paths in order to propose a proper interpretation of the test data. The void ratio and water content stress paths are shown for the situation where there is a minimum distur- bance due to sampling. Even so, the loading path will dis- play some curvature as the net normal stress plane is ap- proached. In reality, the actual stress path will be even more affected by sampling (Fig. 13.33). Geotechnical engineers have long recognized the effect of sample disturbance when determining the preconsoli- dation pressure for a saturated clay. In the aedometer test, it is impossible for the soil specimen to return to an in situ stress state after sampling without displaying some curva- ture in the void ratio versus effective stress plot (i.e., con- solidation curve). However, only recently has the signifi- cance of sampling disturbance been recognized in the measurement of swelling pressure (Fredlund et af., 1980). Sampling disturbance causes the conventionally deter- mined swelling pressure, P, , to fall well below the “ideal” or “correct” swelling pressure, Pi. The “corrected” swelling pressure represents the in siru stress state trans- lated to the net normal stress plane. The “corrected” swelling pressure is equal to the overburden pressure plus the in situ matric suction translated onto the net normal stress plane. The translated in situ matric suction is called the “matric suction equivalent,” (u, - u,,,)~ (Yoshida et af., 1982.) The magnitude of the matric suction equivalent will be equal to or lower than the in situ matric suction. The difference between the in situ matric suction and the matric suction equivalent is primarily a function of the de- gree of saturation of the soil. The engineer desires to obtain the ‘‘corrected” swelling pressure from an oedometer test in order to reconstruct the in siru stress conditions. The procedure to accounting for sampling disturbance is dis- cussed later. %ree-Swell’ ’ Test In the “free-swell” type of oedometer test, the specimen is initially allowed to swell fmly, with only a token load applied (Fig. 13.2(a) and Fig. 13.34). The load required to bring the specimen back to its original void ratio is termed the swelling pressure. The stress paths being fol- lowed can best be understood using a three-dimensional plot of the stress variables versus void ratio and water con- tent, as shown in Fig. 13.34. This test has the limitation that it allows volume change and incorporates hysteresis into the estimation of the in situ stress state. On the other hand, this testing procedure somewhat compensates for the effect of sampling disturbance. Correction for the Compressibility of the Apparatus The following procedure is suggested for obtaining the “corrected” swelling pressure from “constant volume” D Metric suction, (u. - I&,) Figure 13.32 “Ideal” stress path representation for a “constant volume” oedometer test. 390 13 MEASUREMENTS OF VOLUME CHANGE INDICES Ideal stress - deformation path -Actual stress - deformation path * P; (corrected swelling Matric suction, (u. - u,) / I pressure) /.,a *Assume no volume change during sampling Figure 13.33 Ideal and actual stress-deformation paths showing the effect of sampling distuqb- ance. oedometer test results. Detailed testing procedures are pre- sented in ASTM D4546. When interpreting the laboratory data, an adjustment should be made to the data in order to account for the compressibility of the oedometer apparatus. Desiccated, swelling soils have a low Compressibility, and the compressibility of the apparatus can significantly affect the evaluation of in situ stresses and the slope of the re- bound curve (Fredlund, 1969). Because of the low compressibility of the soil, the com- pressibility of the apparatus should be measured using a steel plug substituted for the soil specimen. The measured deflections should be subtracted from the deflections mea- sured when testing the soil. Figure 13.35 shows the manner in which an adjustment should be applied to the laboratory data. The adjusted void ratio versus pressure curve can be Void ratio pressure ‘\ Water content Figure 13.34 Stress path representation for the “free-swell” type of oedometer test. sketched by drawing a horizontal line from the initial void ratio, which curves downward and joins the recompression curve adjusted for the compressibility of the apparatus. Correction for hpling Disturbance Second, a comtion can now be applied for sampling dis- turbance. Sampling disturbance increases the compressi- bility of the soil, and does not permit the laboratory spec- imen to return to its in situ state of stress at its in situ void ratio. Casagrande (1936) proposed an empirical construc- tion on the laboratory curve to account for the effect of Uncorrected Sketched swelling -connecting pressure, P. portion t Test data adjusted of oedometer Log (0 - u.) - Figure 13.35 Adjustment of laboratory test data for the com- pressibility of the oedometer apparatus. 13.2 TEST PROCEDURES AND EQUIPMENTS 391 Regina clay Liquid limit = 75% Depth range = 0.75m - 5.3m No. of oedometer test = 34 sampling disturbance when assessing the preconsolidation pressure of a soil. Other construction procedures have also been proposed (Schmertmann, 1955). A modification of Casagrande’s construction is suggested for determining the “corrected” swelling pressure. The following procedure is suggested for the determi- nation of the “corrected” swelling pressure. Locate the point of maximum curvature where the void ratio versus pressure curve bends downward onto the recompression branch (Fig. 13.36). At the point of maximum curvature, a horizontal line and a tangential line are drawn. The “cor- rected” swelling pressure is designated as the intersection of the bisector of the angle formed by these lines and a line parallel to the slope of the rebound curve which is placed in a position tangent to the loading curve. The need for applying a correction to the swelling pres- sure measured in the laboratory, is revealed in numerous ways. First, it would be anticipated that such a correction is necessary as a result of early experience in determining the preconsolidation pressure for normally consolidated soils. bond, attempts to use the “uncorrected” swelling pressure in the prediction of total heave commonly result in predictions which are too low. Predictions using “cor- rected” swelling pressures may often be twice the magni- tude of those computed when no correction is applied. Third, the analysis of oedometer results from desiccated deposits often produces results which are difficult to inter- pret if no correction is applied for sampling disturbance. Figure 13.37 shows an average oedometer curve ob- tained from 34 tests performed on Regina clay. The deposit is of preglacial lacustrine origin, and the natural water con- tents are near the plastic limit (Fredlund et al., 1980). The average liquid limit is 75 96. The climate of the region is semi-arid, and there is no evidence of a regional gmund- water table in the deposit. The soil is very stiff, and would be anticipated to have high swelling pressures. The oed- ometer results show, however, that if a correction for sam- Uncorrected Corrected Log (a - u.) - Figure 13.36 Constmction pmedure to correct for the effect of sampling disturbance. 1.02 0.98 0.94 6 5 0.90 E 8 0.86 0.82 J 100 lo00 0.78t0 ’ ’ ’””~‘ Log (a, - uw) Wa) Figure 13.37 Average data from dometer tests on Regina clay illustrating the need for the swelling pressure correction. pling disturbance is not applied, the swelling pressure is only slightly in excess of the average overburden pressure. This soil could easily be misinterpreted as a low swelling clay. However, swelling problems are common, with a to- tal heave in the order of 5-15 cm. Samples ‘from depths deeper than 5.5 m often show “uncorrected” swelling pressures less than the overburden pressure. In other words, the correction for sampling disturbance is imperative to the interpretation of the in situ stress state of the soil. Figure 13.38 shows a comparison of “corrected” and “uncorrected” swelling pressure data from two soil de- posits. The results indicate that it is possible for the “cor- loo00 - * Compacted Regina clay 5 % g F f I! 100 H s ’ p“ !! - 10 10 100 lo00 Uncorrected swelling pressure, P. (kPa) Figure 13.38 Change in swelling pressure due to applying the correction for sampling disturbance. [...]... localized increases in soil water content Defective rain gutters and downspouts contribute to localized increases in soil water content 5 ) Seepage into foundation subsoils at soil/ foundation interfaces and through excavations made for basements or shaft foundations leads to increased soil water content beneath the foundation Drying of exposed foundation soils in excavations and reduction in soil surcharge... stress state variables The properties of the soil have been shown to influence the volume change indices For example, soils compacted at various densities and water contents produce different soil structures which have different volume change indices In addition, various densities and water contents also affect the magnitude of the stress state of compacted soils Soils compacted at low densities and high... an unsaturated clay deposit indicate that the soil has a high swelling potential Unsatu, rated soils with a high swelling index, C , in a changing environment are referred to as highly swelling and shrinking soils (i.e., expansive soils) rnl \ / \ loading Overburden,/ \ Evaporation and evapotranspiration L M a t r i c suction, (u - u,) , c - pressure Figure 14.14 Stress state representation when soil. .. ratio of the soil layer , efi = final void ratio of the soil layer 4 08 14 VOLUME CHANGE PREDICTIONS The change in void ratio, dei, in Eq (14.5) can be rewritten, by incorporating the soil properties and the stress states [Le., Eq (14.2)], to give the following form for the heave of a soil layer: (14.6) where Pfi = final stress state in the soil layer Poi = initial stress state in the soil layer The... be 86 % of that anticipated for the case of no excavation However, if the excavated portion is backfilled with a nonexpansive soil, the total heave would be only 15%of 100 i s $15 - 80 60 6 A 3 I 40 ala Y 20 0 2i - J i=l 0 20 40 60 80 100 Figure 14.33 Ratio of total heave for partial excavation and backfilling to total heave for wetting of the entire active depth 14.5 NOTES ON 82 6 ' 14.5 NOTES 5 80 ... Possible remedial measures for heave reduction by flooding, ON COLLAPSIBLE SOILS An expansive soil exhibits a volume increase as a result of a reduction in matric suction, while a collapsible soil exhibits a volume decrease as a result of a reduction in matric suction Collapsible soils have an open type of structure, with large void spaces which give rise to a metastable structure Soils compacted dry of... collapse behavior In general, collapsible soils are unsaturated, and the reduction in matric suction is one of the major causes for the occurrence of collapse (Matyas and Radhakrishna, 19 68; Escario and Sdez, 1973; Cox, 19 78; Lloret and Alonso, 1 980 ; Maswoswe, 1 985 ) Tadepalli (1990) conducted collapse tests in a specially designed oedometerwith matric suction measurements Soil specimens were statically compacted... environmental changes In collapsible soils, the collapse phenomenon occurs when the matric suction of the soil decreases In this chapter, the methodology for the prediction of heave in a swelling soil is described in detail The stress history of a soil is an important factor to consider in understanding the swelling behavior The formulations and example problems for heave prediction are presented and... (kPa) 1 333 500 83 3 80 0 6 08 300 2 3 Initial Change in dterhurden -total stress pressure A u (kPa) uv(kPa) 9 ot6.0 16.4 t6.0 28. 4 t6.0 Final pore-water pressure u ~ f -7.0 -7.0 -7.0 PI = Ah (mm) 22.0 60.6 u f Au - u ~ f , 29.4 41.4 76.7 83 .6 - Total heave= 220.9mm Figure 14.20 Total heave calculations for example no 2 410 14 VOLUME CHANGE PREDICTIONS depth for a 4 m layer of expansive soil The results... the amount of heave is also included At the end, there is a brief note on collapsible soils and methods to predict the amount of collapse 14.1 LITERATURE REVIEW Expansive soils are found in many parts of the world, particularly in semi-arid areas An expansive soil is generally unsaturated due to desiccation Expansive soils also contain clay minerals that exhibit high volume change upon wetting The large . of an unsaturated soil. These tests can be performed using con- ventional soil mechanics testing procedures. The test re- sults give rise to the volume change relationships for an unsaturated. Chapter 12. Soil- water characteristic curves obtained from pressure plate tests are an important part of the water phase consti- tutive surface for an unsaturated soil. An unsaturated soil in. suction for disturbed soil specimens. Curve G is not a soil- water characteristic curve since the points on the curve were obtained from soil specimens with different soil structures. A soil- water